This application seeks to investigate the mechanisms underlying cellular reprogramming, i.e. the conversion of adult cells into pluripotent stem cells. We will utilize transcription factor-mediated reprogramming into induced pluripotent stem cells (iPSCs) as a tool to pursue three major Aims. In Aim 1, we will (i) test whether iPSCs derived from fibroblasts, hematopoietic and myogenic cells are transcriptionally, epigenetically and functionally distinct, (ii) study the mechanism for the increased reprogramming efficiency of progenitors, and (iii) determine if iPSC formation introduces genetic mutations into cells. Addressing the epigenetic and genetic integrity of iPSCs will be crucial for any potential therapeutic applications of this technology and may identify the most suitable cell type for generating patient-specific iPSCs. In Aim 2, we will (i) map the transcriptional and epigenetic changes in intermediate cell populations undergoing reprogramming, (ii) test if reprogramming differentiated cells into pluripotent cells recapitulates stages of normal development, and (iii) perform a gain and loss-of-function screen, respectively, to identify novel regulators of reprogramming. This aim will identify new molecules important during reprogramming, whose manipulation may facilitate the efficient and safe generation of patient-specific iPSCs. In Aim 3, we will investigate the functionality of iPSCs compared with ESCs using in vitro and in vivo assays. Specifically, we will (i) assess whether neural stem cells and fibroblasts derived from iPSCs and ESCs show similar growth and differentiation characteristics in vitro, (ii) test if iPSC-derived hematopoietic stem cells are as potent as ESC-derived hematopoietic stem cells upon serial bone marrow transplantation, and (iii) produce and age entirely ESC and iPSC-derived mice to test if iPSC-derived animals age prematurely or develop cancer. This aim will assess the safety and long-term consequences of iPSCs-derived mature cells in vivo, a prerequisite for using iPSC technology in human cell therapy.